1. Field of the Invention
This invention relates to ultrasonic transducers, to ultrasonic devices and to ultrasonic detection and measuring apparatus. The invention also relates to a method of converting ultrasonic energy into electrical signals and to methods of detection of ultrasonic waves.
2. Description of the Prior Art
Ultrasonic sensing transducers in use at the present time are piezoelectric elements which have phase sensitivity. Such elements convert the acoustic wave into an electrical signal, which is proportional to the average pressure or strain produced in the piezoelectric element. Consequently, since the output signal is proportional to average pressure, it is affected by phase shift and modulation of the ultrasonic waves. This leads to erroneous outputs. Such detectors therefore can be used accurately for unmodulated ultrasonic waves produced by test samples of simple shape, using either echo pulses or transmitted waves. However, it is increasingly desired to use ultrasonic testing in non-destructive evaluation of objects of more complex shape and also in biological and medical fields. Phase-sensitive transducers are inadequate, since they produce erroneous signals if two phase-shifted waves are present simultaneously or if the wave is modulated.
A proposal has been made for a phase-insensitive ultrasonic transducer, using cadmium sulphide as a semiconducting acoustoelectric transducer employing charge carriers which couple to the acoustic wave (U.S. Pat. No. 4,195,244 and a related article "Phase insensitive acoustoelectric transducer" Joseph F. Heyman, J. Acoust. Soc. Am. 64(1), July 1978). These references also discuss earlier articles, devoted to the theory of ultrasonic wave propagation and attenuation in piezoelectric semiconductors. Reference should be made to these prior art documents for further explanation of the acoustoelectric effect. These references specifically mention that CdS is known as a photoconductive transducer, employing photo-generated charge carriers. A major defect of an acoustoelectric transducer relying on the photoconductive effect is the requirement for a light source which is cumbersome and lacks sufficient reliability to provide an accurate output from the transducer. The light source is also a source of electrical noise. The references mentioned appear to suggest that a cadmium sulphide crystal can act as an ultrasonic transducer by absorption of the acoustic energy by the free charge carriers in the crystal, but this is stated to require careful annealing for a particular time and at a particular temperature, to provide the maximum acoustic attenuation at the operating frequency. Such a device has low sensitivity and is specific to a given wavelength.
Further prior art which forms background of the present invention is discussed below, following an explanation of the invention itself.